Image-guided or image-assisted) procedures are medical procedures that require precise insertion or placement of an object within a subject. Such procedures (also referred to as ‘interventions’) are often percutaneous interventions, and the inserted object can be a medical device or instrument such as, for example, a catheter, an electrode, a needle, or the like. Percutaneous interventions can be used for biopsies, thermal ablations, infiltrations, etc. of a target structure. Imaging techniques for such procedures can include, e.g., ultrasound, x-ray, or magnetic resonance (MR) imaging.
For real-time MR-guided needle placement (e.g. biopsy, infiltration, placement of a thermal applicator such as a radio frequency (RF) ablation needle), it is essential to continuously monitor the needle, surrounding sensitive structures and the target structure. Thus, fast image acquisition is needed, e.g., at a rate of several frames per second (fps). A selection of different soft-tissue contrasts is also desirable. For example, some tumors can only be seen in a highly spin-spin relaxation time-weighted (T2-weighted) image. A high spatial resolution is advantageous e.g., when addressing small target structures such as small lesions and the like. However, T2-weighted imaging typically requires a long repetition time TR, and acquisition times for a single frame/image can be on the order of several seconds. Some ‘fast’ imaging methods, such as turbo spin-echo (TSE) sequences, also have acquisition times that are typically more than a second for a single image. Further, there is generally a trade-off between acquisition time and spatial resolution, with higher image resolutions requiring longer acquisition times. A summary of imaging requirements for interventional MRI procedures, and some proposed approaches to improve speed and efficiency of image acquisition for such procedures, is provided, e.g., in J. L. Duerk et al., Journal of Magnetic Resonance Imaging 6(6), 918-924 (1996), which is incorporated herein by reference in its entirety.
Typically, fast imaging sequences such as spin-lattice relaxation time-weighted (T1-weighted) gradient-recalled echo (GRE) or FISP (fast imaging with steady state precession), e.g., True-FISP (a balanced FISP technique in which the magnetic gradient moments sum to zero for each TR), are used for real-time MM-guided needle placement procedures. Such MRI techniques can provide near-continuous imaging, e.g., at several frames per second, while the needle is advanced within a subject. However, obtaining highly-resolved verification image datasets in- between small needle advancements is needed to verify the exact needle position before it is advanced further. These higher-resolution verification datasets often have an acquisition time of 20 seconds or more, and thus slow down the needle positioning procedure. The workflow interruptions needed to accurately verify needle positions during the advancement process slow down the overall procedure and impede a continuous insertion process.
Accordingly, it would be desirable to have a system and method for real-time MRI suitable for guiding the insertion or placement of medical devices (such as needles or catheters) within a subject, which addresses some of the shortcomings described above.